Control module for single 3/2 solenoid controlled relay valve

ABSTRACT

In one embodiment, a vehicle braking system, for delivering pressurized air to a brake chamber to achieve a desired braking response, includes an air-pressure controlled relay valve for delivering the pressurized air to the brake chamber. A solenoid receives a variable control input pressure and delivers the control input pressure to the relay valve as a function of a state of the solenoid. An ECU controls the solenoid according to a control model for delivering the pressurized air to the brake chamber and achieving the desired braking response.

This application is a continuation-in-part of U.S. application Ser. No.10/745,126, filed Dec. 23, 2003, now U.S. Pat. No. 7,020,551.

BACKGROUND

The present invention relates to electronically controlled pressuresystems. It finds particular application in conjunction with controlmodels for controlling pressure in pneumatically operated vehiclessystems and will be described with particular reference thereto. It willbe appreciated, however, that the invention is also amenable to otherapplications.

Electronic controlled brake systems (ECBS or EBS), antilock brakingsystems (ABS), and automatic traction control (ATC) systems areincorporated into vehicles to improve braking performance and vehiclehandling. EBS permits perpetual optimal balancing of braking forcesamong individual wheel brakes and for achieving optimal stability andbraking performance during all driving and braking situations.Proportional-solenoid or multiple-solenoid controlled valves are usedfor distributing and modulating desired pressure to the individualwheels as processed and controlled by an electronic control unit (ECU).The ECU receives sensor input signals from, for example, the driver'sbrake pedal demand, the speed of individual wheel(s), along with controland brake chamber pressures.

The brake pressures calculated by the ECU for the individual wheels mustbe delivered to the respective wheels with a high accuracy and,furthermore, must be delivered and adjusted very quickly. Therefore,pressure modulator relay valves are used in air-braked systems toachieve quick pressure apply and release times. Also, additionalpressure sensors are used to achieve desired pressure accuracies.

A modular relay valve (MRV) operates as a remote controlled brake valvefor delivering or releasing air to brake chambers in response to controlair that is delivered from the driver's operated brake valve or othersources. The relay valve applies, holds, or releases a brake chamber'spressure in proportion to the control pressure, which is controlled as afunction of the driver's brake pedal demand.

ABS and ATC as integrated in EBS prevents wheel lock-ups during brakingand excessive wheel spinning during accelerating in order to providevehicle stability and braking and traction performance.

MRVs used in conjunction with EBS typically include three (3) solenoidsfor controlling the air pressure. A backup solenoid (electrically)provides supply pressures from an air reservoir; a hold solenoidmaintains air pressure; and a release solenoid removes or exhausts airpressure. An MRV used in conjunction with EBS may only include a singlesolenoid, which is designed as a pressure/current proportional solenoid.

A proportional solenoid converts a control current, which is determinedby an algorithm in the ECU, into a proportional control pressure for therelay valve. One advantage of controlling pressure with a proportionalsolenoid is the possibility of providing and controlling differentpressure curves and pressure modulations as a function of the controlcurrent supplied to the solenoid. However, proportional solenoids aremore complex and expensive and, furthermore, require the ECU to supply acurrent controlled output stage that acts as the control current.

SUMMARY

In one embodiment, a vehicle braking system, for delivering pressurizedair to a brake chamber to achieve a desired braking response, includesan air-pressure controlled relay valve for delivering the pressurizedair to the brake chamber. A solenoid receives a variable control inputpressure and delivers the control input pressure to the relay valve as afunction of a state of the solenoid. An ECU controls the solenoidaccording to a control model for delivering the pressurized air to thebrake chamber and achieving the desired braking response.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings which are incorporated in and constitute apart of the specification, embodiments of the invention are illustrated,which, together with a general description of the invention given above,and the detailed description given below, serve to exemplify theembodiments of this invention.

FIG. 1 illustrates a brake circuit incorporating a solenoid used inconjunction with a control model in one embodiment of the presentinvention;

FIG. 2 illustrates a characteristic ABS pressure cycle of a single3/2-solenoid operated relay valve when controlled by a control model inone embodiment of the present invention;

FIG. 3 illustrates experimental data showing a correlation of a dutycycle ratio to a delivery pressure for different variable controlpressure values;

FIG. 4 illustrates a simple mathematical data model showing acorrelation of a duty cycle ratio to a delivery pressure for differentcontrol pressure values;

FIG. 5 illustrates a flow chart of an operation of a pressure estimationprogram in one embodiment of the present invention;

FIG. 6 illustrates a characteristic wheel speed loop of an ABScontrolled wheel in an ABS event and the reacting brake pressurebehavior when controlled with a single 3/2-solenoid relay valve in oneembodiment of the present invention compared with a prior art system;and

FIG. 7 illustrates a graph depicting a controllability of a 3/2-singlesolenoid controlled relay valve in conjunction with a control model inone embodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENT

With reference to FIG. 1, a solenoid controlled relay valve 10 is usedfor controlling air pressure delivered to a system (e.g., an ABScontrol) in conjunction with a control model in one embodiment of thepresent invention. The relay valve 10 includes a solenoid valve section12 and a relay valve section 14.

In one embodiment, the solenoid valve section 12 includes a single3/2-solenoid valve; however, other types of solenoid valves are alsocontemplated. The illustrated 3/2-solenoid valve includes three (3)pneumatic ports (connections) 16, 18, 20 and a switching means 24. Theport 16 is a solenoid valve inlet (control) port; the port 18 is asolenoid valve outlet (delivery) port; and the port 20 is a solenoidvalve exhaust (vent) port. The illustrated switching means 24 is asolenoid switching device including a coil 26, which is capable ofcarrying an electric current, and a magnetic core 30.

The control port 16 fluidly communicates with a brake valve (brakepedal) 32 operated by a vehicle operator. The brake valve 32 alsofluidly communicates with a reservoir 34 storing pressurized fluid(e.g., air). The relay valve 14 transmits the pressurized fluid from thereservoir 34 to a brake chamber (pressure actuator) 38 for controlling aservice brake (not shown) used for decelerating a wheel. An electroniccontrol unit (ECU) 40 monitors a speed of the wheel and at least oneother wheel (not shown). As described in more detail below, thepressurized fluid is transmitted from the reservoir 34 to the brakechamber 38 as a function of how much pressure the vehicle operatorapplies to the brake valve 32 and electrical signals transmitted fromthe ECU 40 to the switching means 24 as a function of the speed of thewheel.

A spring 42 biases the core 30 in a first position (as illustrated inFIG. 1). Therefore, when no electric current is passing through the coil26, the core 30 is in the first position, which is referred to as adeenergized position. While in the deenergized position, the core 30sealingly covers the exhaust port 20 for preventing pressurized air frompassing from the solenoid valve 12 to atmosphere. Passing sufficientelectrical current through the coil 26 causes the core 30 to overcomethe bias of the spring 42 and, furthermore, causes the core 30 to moveto a second position, which is referred to as an energized position.While in the energized position, the core 30 sealingly covers thecontrol port 16, but no longer sealingly covers the exhaust port 20.

The relay valve section 14 includes a relay piston 50 including anexhaust passage 52 that fluidly communicates with the solenoid valveexhaust port 20 and a relay valve exhaust port 54. A control side 56 ofthe piston 50 fluidly communicates with the delivery port 18 of thesolenoid valve section 12. A relay spring 58 biases the relay piston 50in a raised position (as illustrated in FIG. 1). Pressurized fluid fromthe reservoir 34 is fluidly communicated to a relay valve supply port64. A pressurized fluid seal 66 is biased by a spring 68 to sealinglyengage points 70, 72 of the relay valve section 14 for preventing thepressurized fluid communicated from the reservoir 34 from escaping.

A relay valve delivery port 74 fluidly communicates with the brakechamber 38. Furthermore, when the piston 50 is in the raised position(as illustrated in FIG. 1), the brake chamber 38 fluidly communicateswith a relay valve exhaust port 76. Therefore, while in the raisedposition shown in FIG. 1, the relay piston 50 is referred to as being inthe exhaust position.

As the piston 50 moves in a downward direction, a bottom point 80 of thepiston 50 sealingly engages a top portion of the pressurized fluid seal66 for creating a seal between the brake chamber 38 and the relay valvesection 14 exhaust port 76. Furthermore, as the relay piston continuesto move downward, the bias of the storage volume spring 68 is overcomeand the pressurized fluid seal 66 also begins to move in a downwarddirection. As the pressurized fluid seal 66 moves downward, the seal atthe point 70 is broken. As described in more detail below, the positionand movement of the piston 50 is controlled as a function of thepressure on the control side 56 of the piston 50 and in the deliveryport 18 of the solenoid valve section 12.

Once the seal at the point 70 is broken, the brake chamber 38 fluidlycommunicates with the fluid storage volume 60 as a function of a size ofan opening between the point 70 and the pressurized fluid seal 66.Therefore, a pressure in the brake chamber 38 is determined as afunction of the pressure of the pressurized fluid and the size of theopening between the point 70 and the pressurized fluid seal 66.

As discussed above, the position and movement of the piston 50 iscontrolled as a function of the pressure on the control side 56 of thepiston 50 and in the delivery port 18 of the solenoid valve section 12.More specifically, pressurized air is delivered from the air reservoir34 to the solenoid valve control port 16 as a function of a position ofthe brake valve 32. The position of the brake valve 32 is controlled bythe vehicle operator. For example, when it is desired to apply theservice brakes, the operator depresses the brake valve 32. Furthermore,the desired level of service braking pressure applied is proportional toan amount the brake valve 32 is depressed.

During normal braking conditions, the ECU 40 causes the core 30 to bemaintained in the deenergized position. When the core 30 is in thedeenergized position, the pressurized fluid is transmitted between theair reservoir 34 and the control side 56 of the piston 50 via thesolenoid valve control port 16 as a function of the amount the brakevalve is depressed. Once enough pressure builds on the control side 56of the piston 50 to overcome the force of the relay spring 58, thepiston 50 begins to move downward. Furthermore, an amount the piston 50moves downward is proportional to the amount of pressure on the controlside 56 of the piston 50. As discussed above, once enough pressurebuilds on the control side 56 of the piston 50, the piston 50 is moveddownward enough to contact and move the pressurized fluid seal 66 forcausing the brake chamber 38 to communicate with the pressurized fluidas a function of a size of the opening between the point 70 and thepressurized fluid seal 66. As the brake valve is released, thepressurized fluid is released from the control side 56 of the piston 50via the solenoid valve control port 16.

When it is desired to control the pressurized air delivered to the brakechamber 38 in a manner that is not proportional to an amount the brakevalve 32 is depressed (e.g., when a control system such as an anti-lockbraking system (ABS) is activated), delivery of the control pressure tothe relay valve 14 is regulated by actuation and deactuation of thesolenoid 24. The ECU 40 controls the actuation and deactuation of thesolenoid 24 according to a predetermined model. In one embodiment, themodel is stored internally in the ECU 40 and calls for the solenoid 24to be alternately actuated and deactuated.

With reference to FIGS. 1 and 2, a first timing diagram (curve) 82 of acontrol pressure delivered to the control side 56 of the relay piston50, when the solenoid is operated according to the control model in oneembodiment of the present invention, is illustrated. A second timingdiagram (curve) 84 illustrates pressure delivered from the relay valvedelivery port 74 to the actuator 38, when the solenoid is operatedaccording to the control model. The (delivery) pressure cycle 84 in thecontrol model according to one embodiment of the present inventionincludes a first pressure apply time period 86, a first pressure releasetime period 88, a first steady pressure hold time period 90, a secondpressure apply time period 92, an increasing pressure hold time period94, a second pressure release time period 96, and a second steadypressure hold time period 98. A third timing diagram (curve) 100 depictsactivation and deactivation (deenergized) of the solenoid 24 during thevarious time periods 86, 88, 90, 92, 94, 96, 98.

In one embodiment, the control model is stored in the ECU 40 and isdesigned for controlling the solenoid 24 and the actuator 38 during anABS event. However, it is to be understood that other control models,for producing other timing diagrams and delivering pressurized air forother purposes, are also contemplated.

For simplicity, the appropriate wheel speed of the ABS controlled wheelis not shown. However, it is to be understood that there is acorrelation between wheel speed and the pressure periods describedabove. Brake pressure is released in the appropriated brake actuator(chamber) when the controlled wheel is over braked and tends to lock.Brake pressure is held when the speed of the wheel is recovering fromthe locking tendency. If the wheel recovers and is close to the vehiclespeed, the brake pressure builds up again to achieve the optimal brakeperformance and stability of the wheel. FIG. 6, which is described inmore detail below, illustrates a wheel speed and a correspondingreaction brake pressure.

During an event when the solenoid is operated according to the controlmodel, the ECU 40 deenergizes the solenoid 24 during the first timeperiod 86 and, therefore, the control port 16 is open. The controlpressure, which is regulated as a function of how much pressure thedriver applies to the brake valve 32, builds-up through the open controlport 16 and passes to the control side 56 of the relay valve piston 50.When the pressure on the control side 56 of the relay valve 14 exerts aforce great enough to overcome the bias created by the spring 58, therelay piston 50 begins moving downward. If the pressure on the controlside 56 of the relay valve 14 is great enough to move the relay piston50 such that the point 80 sealingly engages and moves the seal 66downward, supply pressure passes from the storage volume 34 to the brakechamber 38 as a function of the size of the opening. Therefore, thepressure in the brake chamber 38 is proportional to the pressure at thecontrol port 16 of the solenoid valve. It is to be understood that thereis a time delay between changes in the pressure at the control port 16and a corresponding change in pressure at the delivery port 74 of therelay valve 14. The time delay is due to hysteresis of the relay valve.

During the time period 88, the switching means 24 is energized.Therefore, the pressure on the control side 56 of the piston 50 isexhausted via the passage 52.

During the time period 90, a pressure hold phase is required and thesolenoid 24 is alternately energized and deenergized by the ECU 40according to the internally stored control model. By alternatingactivation of the solenoid 24 between build and exhaust phases at agiven duty cycle, a pulsated up and down control pressure is built onthe control side 56 of the piston 50. But due to the hysteresis and slowresponse of the piston 50, the resulting position of the piston 50 isdetermined as the average of the modulated pressure on the control side56 of the piston. In this sense, the modulated pressure on the controlside of the piston 50 is quasi-filtered and leveled by the hysteresisand, furthermore, the resulting brake chamber 38 pressure isproportional to the average pressure (without any overshot) on thecontrol side 56. To achieve a hold phase (e.g., during the time period90), the frequency and the percentages of the duty cycle ratio of thealternating energizing and deenergizing of the solenoid 24 are constant.

During the time period 92, the solenoid 24 is deenergized and deliverypressure builds up in a similar manner as during the time period 86.

During the time period 94, the ECU 40 again alternately energizes anddeenergizes the solenoid to obtain a similar reaction and response asdescribed above for time period 90. Although the frequency of thealternating activation of the solenoid 24 is the same as in the timeperiod 90, the duty cycle ratio is slowly changed (e.g., decreased) bythe ECU 40 during the time period 94. Decreasing the duty cycle ratio bya constant frequency means changing the ratio of energized todeenergized time in that way that a higher average of the pressure onthe control side 56 is achieved. Consequently, the pressure at thedelivery port 74 increases in a proportional manner.

During the time period 96, the solenoid 24 is energized for resulting ina similar decline in the pressure at the delivery port 74 as achieved inthe time period 88.

During the time period 98, the solenoid 24 is energized and deenergizedfor resulting in a similar hold in the pressure at the delivery port 74as achieved in the time period 90.

With reference to FIGS. 1 and 3, a graph 110 depicts the duty cycleratio Φ (x-axis) to a dedicated delivery pressure (y-axis) in dependencyto the variable control pressure. The diagram as shown is based mainlyon experimental data. The duty cycle ratio in % is defined asΦ=(T_(E)/T_(E)+T_(B))*100 where T_(E)=Exhaust Time (solenoid 24 isenergized for pressure release) and T_(B)=Build Time (solenoid 24 is notenergized and pressure builds up). The frequency f of the duty cycleperiod is f =1/T_(E)+T_(B) and is determined by the limit at the low endof a still acceptable ripple on delivery pressure and at the high end ofthe death or lifetime of the solenoid itself.

The reaction time and the hysteresis of the solenoid 24 and relay piston50 also determine the low-end frequency. The frequency in theillustrated graph 110 is 25 Hz. A lower frequency provides a morewavelike delivery pressure and a higher frequency increases the numberof solenoid activation's and, consequently, decreases the lifetime ofthe solenoid. However, it is to be understood that other solenoid and/orrelay valve designs requiring different possible frequency ranges arealso contemplated.

According to the graph 110, a desired pressure may be delivered to thedelivery port 74 with a particular duty cycle ratio of the activation ofthe solenoid 24 for a given control pressure. The frequency in this casefrom switching on and of the solenoid 24 is constant and only the length(percentage) of energized phases (TE) to the length of not energizingphase (T_(B)) is varied.

As an example, the graph 110 illustrates that for a given controlpressure of 80 psi a desired pressure of 39 psi is delivered with a 60%duty cycle ratio activation of the solenoid 24. In other words, 60% ofthe time T_(E)+T_(B) the solenoid is energized (e.g., in the pressurerelease mode) and 40% of time T_(E)+T_(B) the solenoid is not energized(e.g., pressure is building up). With the same duty cycle ratio of 60% apressure of 60 psi is delivered with a 120-psi control pressure value.

Like the graph 110, a graph 112 shown in FIG. 4 depicts the duty cycleratio Φ (x-axis) to a dedicated delivery pressure (y-axis) in dependencyto the variable control pressure. However, while the graph 110 shown inFIG. 3 is based on experimental data, the graph 112 shown in FIG. 4 isbased on a mathematical based program that considers the physicalbehavior of pressure build and release.

A simplified duty cycle data model is be derived from the followingmathematical formulas:

The generalized pressure change rate is for increasing pressure:

$\frac{\mathbb{d}p}{\mathbb{d}t} = {K_{b}\sqrt{\left( {P_{c} - p} \right)}}$

${{and}\mspace{14mu}\frac{\mathbb{d}p}{\mathbb{d}t}} = {{- K_{e}}\sqrt{(p)}}$for decreasing pressure. K_(B) and K_(E) are constants for modeling therestriction control inlet to control volume and control volume toatmosphere.

For small changes in time, the change in control volume pressure (p) maybe approximated by first order expansion. For the nominal non-energizedcase, an increase for any control volume pressure p is:

$\begin{matrix}{{\Delta\; p_{B}} = {\frac{\mathbb{d}p}{\mathbb{d}t}(p)*T_{B}}} & \; & \; & {T_{B} = {{Build}\mspace{14mu}{Time}}}\end{matrix}$

For the energized case, the decrease in control volume pressure is:

$\begin{matrix}{{\Delta\; p_{E}} = {\frac{\mathbb{d}p}{\mathbb{d}t}(p)*T_{E}}} & \; & \; & {T_{E} = {{Exhaust}\mspace{14mu}{{Time}.}}}\end{matrix}$

Duty Cycle Selection

While pulse width modulating the solenoid, the duty cycle Φ is definedas

$\frac{T_{E}}{T_{E} + T_{B}}.$The average pressure within the control volume as a function of Φ can befound by solving for p since the steady state pressure level is thepressure where build and exhaust phases effectively cancel.

$\begin{matrix}{{K_{B}\sqrt{\left( {P_{c} - p} \right)}\left( {1 - \Phi} \right)} = {K_{E}\sqrt{(p)}(\Phi)}} \\{\frac{p}{P_{c}} = \frac{K_{B}^{2}}{\left( {{K_{E}^{2}\Phi^{2}} + {K_{B}^{2}\left( {1 - \Phi} \right)}^{2}} \right.}}\end{matrix}$

Comparing this characteristic to experimental data can determine theapproximate ratio. Exact constants are not needed unless transientanalysis is needed.

Period Selection

Selection of the PWM period, T_(E)+T_(B), determines the amount ofripple seen in the control volume. Since Δp_(B)=Δp_(E) at any steadystate control volume pressure p, the peak to peak change is equal to

${\frac{\mathbb{d}p}{\mathbb{d}t}(p)*T_{B}} = {\frac{\mathbb{d}p}{\mathbb{d}t}(p)*{T_{E}.}}$

Increasing, the PWM period while maintaining the same duty cycleincreases this ripple term until the first order expansion is no longervalid. Decreasing the PWM period shows that the ripple can be forcedtoward zero with the only physical limitation being the dead time of thesolenoid itself.

Typically, the load volume controlled by the relay has a significantlyslower dynamic response than the control volume since it is typicallymuch larger. Therefore, the PWM period can be experimentally determineddepending on the load pressure ripple requirements.

This simple mathematical model as shown in FIG. 4 proves to beconsistant with the experimental data as shown in FIG. 3. The model inFIG. 4 does show some discrepencies at the high end of the duty cycleaxis. However, these differences can be attributed to approaching thesolenoid response bandwidth as energized time is minimized or maximized.

FIGS. 3 and 4 depict the delivery pressure is a function of the dutycycle ratio and the control pressure. However, the control pressure asdelivered from the driver operated brake valve is not always known. Forexample, the control as the drivers request by applying the footoperated brake valve is unknown during normal braking and also during anABS event. This handicap may be eliminated with the use of apressure-estimation-program 120, which is illustrated in FIG. 5.

The pressure estimation program 120 is active only during an ABS eventwhen the brake pressure needs to be adjusted by the solenoid controlledrelay valve 14. The program calculates the build-up target pressure forthe subsequent wheel speed cycle of each individual ABS controlledwheel. If the actual pressure in the following wheel speed cycle isdifferent from the previous estimated target pressure, the program makesadjustments for the next wheel speed cycle. With this method of pressureestimation, the unknown control pressure value is automatically takeninto consideration.

As depicted in the flow chart diagram 120 in FIG. 5, the pressureestimation program starts with a pressure demand (p_(start)) as half ofthe maximal possible brake pressure (p_(max)). P_(max) is the airreservoir pressure level and typically approximately 120 psi. Since noinformation of the actual pressure level is available when the firstsolenoid is activated, starting with the first pressure estimation ashalf of the maximum pressure level is a compromise to find the rightpressure level.

From the first release cycle associated with each electrical actuationcommand of the appropriate solenoid, a time adequate pressure level issubtracted when in release mode (Δp_(rel)) and added when in build up(increasing) mode (Δp_(up)). The Δp value is calculated with a formulathat considers the physical behavior of compressed air when released andsubsequently rebuilt and includes the dependence of the pressuregradient to the pressure level and also the activation time of thesolenoid controlled pressure modulator valves. Different Δp-factors areused for pressure release (Δp_(rel)) and for pressure increase(Δp_(up)).

The new estimated brake pressure level p_(new) is increased(p_(old)+p_(up)) during the next pressure build-up phase if the actualbrake pressure reached a higher level than estimated. The new estimatedpressure level p_(new) is decreased (p_(old)−p_(rel)) during the nextpressure build-up phase if the actual brake pressure reached a lowerlevel than estimated. The measurement reading for the correlation of theestimated pressure level to the actual pressure level is the actuationtime of the solenoid. The solenoid actuation time is counted in eachwheel cycle for pressure release and for pressure build up.

The pressure approximation method illustrated in FIG. 5 allows arelative accurate estimation of the instantaneous pressure level duringan ABS event.

FIG. 6 shows a timing diagram 130 illustrating the advanced pressurecontrol possibility and their appropriate wheel speed when the controlmodel as described in one embodiment of the present invention operatesthe solenoid 132 in comparison to the two-solenoid control in the priorart 134.

The characteristic pressure control in an ABS event is to respond to anover braked wheel with a pressure release, waiting with a pressure holdwhen the wheel speed is recovering and build-up pressure again toachieve an optimal brake performance. With the time delay betweencontrol and delivery pressure and slow response of the relay piston,pressure control accuracy is limited when a relay valve is controlledwith two solenoids. This disadvantage is substantially eliminated whenthe solenoid is operated with the control model in one embodiment of thepresent invention (especially during the hold and slow build-up pressurestages).

The solenoid control of the present invention can hold at every pressurelevel and can slowly build-up pressure without any pressureovershooting. This different pressure control behavior is based on theindividual control target demand of both solenoid arrangements.

The control model in one embodiment of the present invention controlsthe solenoid pressure with targeting the delivery pressure value. Thecombined use of the pressure estimation program as described in FIG. 5in conjunction with the duty cycle calculation program as described inFIG. 3 and/or FIG. 4 allows a desired brake pressure to be achieved asdemanded.

The hatched area between the two pressure traces in FIG. 6 illustratesthe difference between the two pressure control method in one embodimentof the present invention (dotted line) and the method of the prior artincorporating a plurality of solenoids (solid line).

The area a illustrates the difference when a hold stage is required at alower pressure level. The two solenoid controlled relay valves willusually release the pressure completely. This is because the controlpressure leads the delivery pressure. The control pressure may be fullyexhausted while the delivery pressure is at the appropriate pressurevalue based upon the wheel speed behavior observed by the ECU. Once thecontrol pressure is completely exhausted, the delivery pressure willcontinue to decrease until it is completely exhausted even if the ECU isdemanding that the system hold at a given pressure. It is difficult tomaintain a small pressure in the delivery after an exhaust has beencommanded. Unlike the system with two solenoids, the single solenoidcontrol is targeting a desired hold pressure value. As shown in the timeperiod 90 in FIG. 2, the intermittently activated solenoid 24 with theappropriate duty cycle ratio overcomes and compensates the pressuredelay between control and delivery pressure.

The same situation is true in the build pressure stages as shown in areab. To avoid overshooting the targeting delivery pressure value, the twosolenoid controlled relay valves of the prior art can only approach thetarget pressure in a step by step fashion. Consequently, under brakingmay result in the beginning of the brake cycle and overbraking mayresult at the end of the brake cycle. The single solenoid with thequasi-linear characteristic of the duty cycle ratio to the deliverypressure, on the other hand, is capable of changing the grade of holdingpressure from a steep grade to a flat grade in a smooth and constantmanner by just changing the duty cycle in the appropriate manner.

The single solenoid, when operated with the control model in oneembodiment of the present invention, produces a more accurate pressurecontrol when compared to the prior art two-solenoid control. It also hasthe capability of providing more optimal control performance. Thedelivery pressure can be at the optimal pressure for a longer timeduring the control event.

As shown in FIG. 6 with the doted pressure line as marked with d, theflat build pressure grade with the single solenoid control is on ahigher average level and extends against the two solenoid controlledpressure as shown with the solid line. With this extension, theappropriate wheel speed can be held longer in the best braking and beststability speed range (marked as e). Next to this performanceenhancement, a lower control cycle frequency is achieved which alsoresults in lower air consumption.

FIG. 7 depicts a control pressure graph 140 along with a graph 142showing a saw tooth shaped delivery pressure, and a graph 144 showingthe solenoid activation stages. The graphs 140, 142, 144 of FIG. 7demonstrate the controllability of a single solenoid controlled relayvalve as shown in FIG. 1. This practical example of the control model inone embodiment of the present invention can be used in an assembly lineto bring production pieces in place.

While the present invention has been illustrated by the description ofembodiments thereof, and while the embodiments have been described inconsiderable detail, it is not the intention of the applicants torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. Therefore, the invention, in its broaderaspects, is not limited to the specific details, the representativeapparatus, and illustrative examples shown and described. Accordingly,departures may be made from such details without departing from thespirit or scope of the applicants' general inventive concept.

1. An antilock braking system, comprising: a relay valve for deliveringpressurized air to a brake actuator; a switching means receiving avariable control input pressure and delivering the control inputpressure to the relay valve as a function of a position of the switchingmeans; and an ECU for controlling the switching means according to acontrol model for delivering the pressurized air to the brake actuatorfor achieving an antilock braking response, the pressure of the airdelivered to the brake actuator changing substantially linearly as afunction of an average of the control input pressures supplied to therelay valve by the switching means alternately changing positions duringa time period.
 2. The antilock braking system as set forth in claim 1,wherein the switching means is a single solenoid.
 3. The antilockbraking system as set forth in claim 1, wherein the solenoid includes: acore switchable between a first position and a second position, thecontrol pressure building on a piston of the relay valve when the coreis in the first position and being exhausted when the core is in asecond position.
 4. The antilock braking system as set forth in claim 3,wherein: the control model causes the ECU to move the core between thetwo positions at a predetermined frequency; and the pressurized airdelivered to the brake actuator is held substantially constant as afunction of an average of the pressures applied to the piston while thecore is moving between the two positions.
 5. The antilock braking systemas set forth in claim 4, wherein: the pressurized air delivered to thebrake actuator is held substantially constant as a function of ahysteresis of the piston.
 6. The antilock braking system as set forth inclaim 1, wherein: the ECU estimates the pressure of the air delivered tothe brake actuator as a function of a previous pressure of the airdelivered to the brake actuator.
 7. A method for delivering pressurizedair to a brake chamber for achieving a desired braking response, themethod comprising: receiving a variable control input pressure into asolenoid; controlling the solenoid to operate between two statesaccording to a control model; delivering the control input pressure to arelay valve as a function of the state of the solenoid; and deliveringthe pressurized air to the brake chamber via the relay valve as afunction of the control input pressure in the relay valve, the pressureof the air delivered to the brake chamber changing substantiallylinearly as a function of an average of the control input pressuressupplied to the relay valve while the solenoid alternately changespositions during a time period.
 8. The method for delivering pressurizedair to a brake chamber as set forth in claim 7, further including:moving a piston to a position in the relay valve according to theaverage of the control input pressure in the relay valve.
 9. The methodfor delivering pressurized air to a brake chamber as set forth in claim8, wherein delivering the pressurized air to the brake chamber includes:delivering the pressurized air to the brake chamber as a function of theposition of the piston in the relay valve.
 10. The method for deliveringpressurized air to a brake chamber as set forth in claim 9, furtherincluding: setting a duty cycle of the solenoid, a substantiallyconstant position of the piston being set as a function of the dutycycle and a hysteresis of the piston.
 11. The method for deliveringpressurized air to a brake chamber as set forth in claim 10, furtherincluding: varying the duty cycle of the solenoid, the substantiallyconstant position of the piston changing linearly as a function of thevariance of the duty cycle.
 12. The method for delivering pressurizedair to a brake chamber as set forth in claim 7, further including:activating an ECU according to the control model for setting thesolenoid in one of the states.
 13. The method for delivering pressurizedair to a brake chamber as set forth in claim 7, further including:setting a duty cycle of the solenoid, a substantially constant deliveryof the pressurized air to the brake chamber being set as a function ofthe duty cycle.
 14. The antilock braking system as set forth in claim 6,wherein: if an actual pressure of the air delivered to the brakeactuator during a current wheel speed cycle is higher than previouslyestimated, the ECU estimates a higher pressure of the air delivered tothe brake actuator during a next wheel speed cycle.
 15. The method fordelivering pressurized air to a brake chamber as set forth in claim 7,further including: estimating the pressure of the air delivered to thebrake chamber as a function of a previous pressure of the air deliveredto the brake chamber.
 16. The method for delivering pressurized air to abrake chamber as set forth in claim 15, further including: if an actualpressure of the air delivered to the brake chamber during a currentwheel speed cycle is higher than previously estimated, estimating ahigher pressure of the air delivered to the brake chamber during a nextwheel speed cycle.